This invention relates to electrocoating of metal containers. More particularly, the invention relates to a dual flow system of electrocoating the interior and exterior of a metal container simultaneously.
Usually, metal containers, such as cans and the like, have their interior surfaces and exterior surfaces coated in separate operations. The interior coating is usually applied at a thicker coating weight than the subsequently applied exterior coating. The thicker interior coating is required for protection of the container from its contents and protection of the contents of the container from reaction with the metal, while the thinner exterior coating may improve handling of the container and/or its aesthetic appearance. Additionally, an exterior coating can offer protection against the container environment, such as by inhibiting the rusting of steel and tin-free steel containers in moist atmospheres and the forming of excessive oxide on aluminum containers during retort.
The coating materials may be applied by sprays, rolls, immersion or the like using conventional polymer systems or by using electrocoating techniques. Electrocoating can provide uniform and consistent films and is a desirable approach. As used herein, electrocoating is the electrodeposition of resinous coating materials, preferably organic, on electrically conductive surface areas from either anodic or cathodic electrocoating material mediums. A layer of particulate coating material is electrodeposited on an electrically charged metal substrate immersed or surrounded in the coating material as an electrical potential is impressed between the substrate and an oppositely electrically charged electrode.
A process and apparatus for uniformly electrocoating a container is disclosed in each of U.S. Pat. Nos. 3,922,213, issued Nov. 25, 1975, and 4,094,760, issued June 13, 1978, to the common assignee of the present invention. The method and apparatus of both patents, the disclosures of which are incorporated herein by reference, considerably improved the prior techniques for electrocoating metal containers by avoiding the slower prior art processes. U.S. Pat. No. 3,922,213 relates to the uniform electrocoating of the interior of a shaped metal container, while U.S. Pat. No. 4,094,760 is an improvement thereover which permits the electrocoating of both the interior and exterior of a metal container simultaneously. Both patents disclose uniformly electrocoating the interior of a metal container which is in an inverted position by the insertion of an electrically conducting probe-nozzle therein through which coating material is flowed into the interior of the container to fill the container and maintain a transient bath of coating material therein. An electrical potential is impressed between the container and the nozzle to coat the metal surfaces. U.S. Pat. No. 4,094,760 also discloses impressing an electrical potential between the container and an outer housing to coat the exterior of the metal container simultaneously.
Though the methods and apparatus described in those patents are suitable for electrocoating metal containers in high speed production lines, there still exists the need for faster electrocoating of the interior and exterior of a metal container simultaneously in one high-speed operation. Improvements in the speed of electrocoating can result from faster filling of the electrocoating cell within which the container is housed during electrocoating than is obtainable by the monoflow system disclosed in the prior patents. Any improvements in speed also should permit application of different coating weights on the interior and exterior surfaces while achieving good coating integrity.